Why is it important to Gatsby that Daisy say she never loved Tom, only him? What is the key sentence that shows Daisy will not leave Tom?

Gatsby always wanted to remake the past. He wanted to strike out the five years they had been apart, like it never existed. Daisy admitting she never loved Tom validates everything that Gatsby had done to win Daisy once more,

But with every word she was drawing further and further into herself, so he gave that up, and only the dead dream fought on as the afternoon slipped away, trying to touch what was no longer tangible, struggling unhappily, undespairingly, toward that lost voice across the room.